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[rustc.git] / src / doc / book / src / ch03-05-control-flow.md

## Control Flow

The ability to run some code depending on if a condition is true, or run some
code repeatedly while a condition is true, are basic building blocks in most
programming languages. The most common constructs that let you control the flow
of execution of Rust code are `if` expressions and loops.

### `if` Expressions

An `if` expression allows you to branch your code depending on conditions. You
provide a condition and then state, “If this condition is met, run this block
of code. If the condition is not met, do not run this block of code.”

Create a new project called *branches* in your *projects* directory to explore
the `if` expression. In the *src/main.rs* file, input the following:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-26-if-true/src/main.rs}}
```

All `if` expressions start with the keyword `if`, followed by a condition. In
this case, the condition checks whether or not the variable `number` has a
value less than 5. We place the block of code to execute if the condition is true
immediately after the condition inside curly brackets. Blocks of code
associated with the conditions in `if` expressions are sometimes called *arms*,
just like the arms in `match` expressions that we discussed in the [“Comparing
the Guess to the Secret Number”][comparing-the-guess-to-the-secret-number]<!--
ignore --> section of Chapter 2.

Optionally, we can also include an `else` expression, which we chose
to do here, to give the program an alternative block of code to execute should
the condition evaluate to false. If you don’t provide an `else` expression and
the condition is false, the program will just skip the `if` block and move on
to the next bit of code.

Try running this code; you should see the following output:

```console
{{#include ../listings/ch03-common-programming-concepts/no-listing-26-if-true/output.txt}}
```

Let’s try changing the value of `number` to a value that makes the condition
`false` to see what happens:

```rust,ignore
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-27-if-false/src/main.rs:here}}
```

Run the program again, and look at the output:

```console
{{#include ../listings/ch03-common-programming-concepts/no-listing-27-if-false/output.txt}}
```

It’s also worth noting that the condition in this code *must* be a `bool`. If
the condition isn’t a `bool`, we’ll get an error. For example, try running the
following code:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-28-if-condition-must-be-bool/src/main.rs}}
```

The `if` condition evaluates to a value of `3` this time, and Rust throws an
error:

```console
{{#include ../listings/ch03-common-programming-concepts/no-listing-28-if-condition-must-be-bool/output.txt}}
```

The error indicates that Rust expected a `bool` but got an integer. Unlike
languages such as Ruby and JavaScript, Rust will not automatically try to
convert non-Boolean types to a Boolean. You must be explicit and always provide
`if` with a Boolean as its condition. If we want the `if` code block to run
only when a number is not equal to `0`, for example, we can change the `if`
expression to the following:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-29-if-not-equal-0/src/main.rs}}
```

Running this code will print `number was something other than zero`.

#### Handling Multiple Conditions with `else if`

You can use multiple conditions by combining `if` and `else` in an `else if`
expression. For example:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-30-else-if/src/main.rs}}
```

This program has four possible paths it can take. After running it, you should
see the following output:

```console
{{#include ../listings/ch03-common-programming-concepts/no-listing-30-else-if/output.txt}}
```

When this program executes, it checks each `if` expression in turn and executes
the first body for which the condition holds true. Note that even though 6 is
divisible by 2, we don’t see the output `number is divisible by 2`, nor do we
see the `number is not divisible by 4, 3, or 2` text from the `else` block.
That’s because Rust only executes the block for the first true condition, and
once it finds one, it doesn’t even check the rest.

Using too many `else if` expressions can clutter your code, so if you have more
than one, you might want to refactor your code. Chapter 6 describes a powerful
Rust branching construct called `match` for these cases.

#### Using `if` in a `let` Statement

Because `if` is an expression, we can use it on the right side of a `let`
statement to assign the outcome to a variable, as in Listing 3-2.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/listing-03-02/src/main.rs}}
```

<span class="caption">Listing 3-2: Assigning the result of an `if` expression
to a variable</span>

The `number` variable will be bound to a value based on the outcome of the `if`
expression. Run this code to see what happens:

```console
{{#include ../listings/ch03-common-programming-concepts/listing-03-02/output.txt}}
```

Remember that blocks of code evaluate to the last expression in them, and
numbers by themselves are also expressions. In this case, the value of the
whole `if` expression depends on which block of code executes. This means the
values that have the potential to be results from each arm of the `if` must be
the same type; in Listing 3-2, the results of both the `if` arm and the `else`
arm were `i32` integers. If the types are mismatched, as in the following
example, we’ll get an error:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-31-arms-must-return-same-type/src/main.rs}}
```

When we try to compile this code, we’ll get an error. The `if` and `else` arms
have value types that are incompatible, and Rust indicates exactly where to
find the problem in the program:

```console
{{#include ../listings/ch03-common-programming-concepts/no-listing-31-arms-must-return-same-type/output.txt}}
```

The expression in the `if` block evaluates to an integer, and the expression in
the `else` block evaluates to a string. This won’t work because variables must
have a single type, and Rust needs to know at compile time what type the
`number` variable is, definitively. Knowing the type of `number` lets the
compiler verify the type is valid everywhere we use `number`. Rust wouldn’t be
able to do that if the type of `number` was only determined at runtime; the
compiler would be more complex and would make fewer guarantees about the code
if it had to keep track of multiple hypothetical types for any variable.

### Repetition with Loops

It’s often useful to execute a block of code more than once. For this task,
Rust provides several *loops*, which will run through the code inside the loop
body to the end and then start immediately back at the beginning. To
experiment with loops, let’s make a new project called *loops*.

Rust has three kinds of loops: `loop`, `while`, and `for`. Let’s try each one.

#### Repeating Code with `loop`

The `loop` keyword tells Rust to execute a block of code over and over again
forever or until you explicitly tell it to stop.

As an example, change the *src/main.rs* file in your *loops* directory to look
like this:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-32-loop/src/main.rs}}
```

When we run this program, we’ll see `again!` printed over and over continuously
until we stop the program manually. Most terminals support the keyboard shortcut
<span class="keystroke">ctrl-c</span> to interrupt a program that is stuck in
a continual loop. Give it a try:

<!-- manual-regeneration
cd listings/ch03-common-programming-concepts/no-listing-32-loop
cargo run
CTRL-C
-->

```console
$ cargo run
   Compiling loops v0.1.0 (file:///projects/loops)
    Finished dev [unoptimized + debuginfo] target(s) in 0.29s
     Running `target/debug/loops`
again!
again!
again!
again!
^Cagain!
```

The symbol `^C` represents where you pressed <span class="keystroke">ctrl-c
</span>. You may or may not see the word `again!` printed after the `^C`,
depending on where the code was in the loop when it received the interrupt
signal.

Fortunately, Rust also provides a way to break out of a loop using code. You
can place the `break` keyword within the loop to tell the program when to stop
executing the loop. Recall that we did this in the guessing game in the
[“Quitting After a Correct Guess”][quitting-after-a-correct-guess]<!-- ignore
--> section of Chapter 2 to exit the program when the user won the game by
guessing the correct number.

We also used `continue` in the guessing game, which in a loop tells the program
to skip over any remaining code in this iteration of the loop and go to the
next iteration.

#### Returning Values from Loops

One of the uses of a `loop` is to retry an operation you know might fail, such
as checking whether a thread has completed its job. You might also need to pass
the result of that operation out of the loop to the rest of your code. To do
this, you can add the value you want returned after the `break` expression you
use to stop the loop; that value will be returned out of the loop so you can
use it, as shown here:

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-33-return-value-from-loop/src/main.rs}}
```

Before the loop, we declare a variable named `counter` and initialize it to
`0`. Then we declare a variable named `result` to hold the value returned from
the loop. On every iteration of the loop, we add `1` to the `counter` variable,
and then check whether the counter is equal to `10`. When it is, we use the
`break` keyword with the value `counter * 2`. After the loop, we use a
semicolon to end the statement that assigns the value to `result`. Finally, we
print the value in `result`, which in this case is 20.

#### Loop Labels to Disambiguate Between Multiple Loops

If you have loops within loops, `break` and `continue` apply to the innermost
loop at that point. You can optionally specify a *loop label* on a loop that we
can then use with `break` or `continue` to specify that those keywords apply to
the labeled loop instead of the innermost loop. Loop labels must begin with a
single quote. Here’s an example with two nested loops:

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-32-5-loop-labels/src/main.rs}}
```

The outer loop has the label `'counting_up`, and it will count up from 0 to 2.
The inner loop without a label counts down from 10 to 9. The first `break` that
doesn’t specify a label will exit the inner loop only. The `break
'counting_up;` statement will exit the outer loop. This code prints:

```console
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-32-5-loop-labels/output.txt}}
```

#### Conditional Loops with `while`

A program will often need to evaluate a condition within a loop. While the
condition is true, the loop runs. When the condition ceases to be true, the
program calls `break`, stopping the loop. It’s possible to implement behavior
like this using a combination of `loop`, `if`, `else`, and `break`; you could
try that now in a program, if you’d like. However, this pattern is so common
that Rust has a built-in language construct for it, called a `while` loop. In
Listing 3-3, we use `while` to loop the program three times, counting down each
time, and then, after the loop, print a message and exit.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/listing-03-03/src/main.rs}}
```

<span class="caption">Listing 3-3: Using a `while` loop to run code while a
condition holds true</span>

This construct eliminates a lot of nesting that would be necessary if you used
`loop`, `if`, `else`, and `break`, and it’s clearer. While a condition holds
true, the code runs; otherwise, it exits the loop.

#### Looping Through a Collection with `for`

You can choose to use the `while` construct to loop over the elements of a
collection, such as an array. For example, the loop in Listing 3-4 prints each
element in the array `a`.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/listing-03-04/src/main.rs}}
```

<span class="caption">Listing 3-4: Looping through each element of a collection
using a `while` loop</span>

Here, the code counts up through the elements in the array. It starts at index
`0`, and then loops until it reaches the final index in the array (that is,
when `index < 5` is no longer true). Running this code will print every element
in the array:

```console
{{#include ../listings/ch03-common-programming-concepts/listing-03-04/output.txt}}
```

All five array values appear in the terminal, as expected. Even though `index`
will reach a value of `5` at some point, the loop stops executing before trying
to fetch a sixth value from the array.

However, this approach is error prone; we could cause the program to panic if
the index value or test condition are incorrect. For example, if you changed
the definition of the `a` array to have four elements but forgot to update the
condition to `while index < 4`, the code would panic. It’s also slow, because
the compiler adds runtime code to perform the conditional check of whether the
index is within the bounds of the array on every iteration through the loop.

As a more concise alternative, you can use a `for` loop and execute some code
for each item in a collection. A `for` loop looks like the code in Listing 3-5.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/listing-03-05/src/main.rs}}
```

<span class="caption">Listing 3-5: Looping through each element of a collection
using a `for` loop</span>

When we run this code, we’ll see the same output as in Listing 3-4. More
importantly, we’ve now increased the safety of the code and eliminated the
chance of bugs that might result from going beyond the end of the array or not
going far enough and missing some items.

Using the `for` loop, you wouldn’t need to remember to change any other code if
you changed the number of values in the array, as you would with the method
used in Listing 3-4.

The safety and conciseness of `for` loops make them the most commonly used loop
construct in Rust. Even in situations in which you want to run some code a
certain number of times, as in the countdown example that used a `while` loop
in Listing 3-3, most Rustaceans would use a `for` loop. The way to do that
would be to use a `Range`, provided by the standard library, which generates
all numbers in sequence starting from one number and ending before another
number.

Here’s what the countdown would look like using a `for` loop and another method
we’ve not yet talked about, `rev`, to reverse the range:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-34-for-range/src/main.rs}}
```

This code is a bit nicer, isn’t it?

## Summary

You made it! That was a sizable chapter: you learned about variables, scalar
and compound data types, functions, comments, `if` expressions, and loops!
To practice with the concepts discussed in this chapter, try building
programs to do the following:

* Convert temperatures between Fahrenheit and Celsius.
* Generate the nth Fibonacci number.
* Print the lyrics to the Christmas carol “The Twelve Days of Christmas,”
  taking advantage of the repetition in the song.

When you’re ready to move on, we’ll talk about a concept in Rust that *doesn’t*
commonly exist in other programming languages: ownership.

[comparing-the-guess-to-the-secret-number]:
ch02-00-guessing-game-tutorial.html#comparing-the-guess-to-the-secret-number
[quitting-after-a-correct-guess]:
ch02-00-guessing-game-tutorial.html#quitting-after-a-correct-guess